I just bought a Used 87 Hobie 17 SE. The lower mainsheet block has an extra pulley attached to the padeye below the cleat. This is different from the manual. Has anyone seen this before? Is this how the Hobie 18 is rigged?

There is a pulley attached to the padeye below the cleat? There would be no reason for that. The padeye below the cleat is used as a fairlead for the mainsheet, it should have nothing attached to it.

In some cases, people add a padeye and pulley ABOVE the cleat so that the mainsheet can be strung 7:1 rather than the stock 6:1.

Just to verify, you have your mainsheet system set up with the ratcheting block connected to the traveler, right, not hanging from the boom? If you have the whole mainsheet system on upside down, then the extra block would be mounted below the cleat...